1. Field of the Invention
The present invention relates to a light source device having increased directivity of emitted light, to a display device that uses this light source device, and to a terminal device.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals have been widely deployed and used in a range of devices that includes monitors, televisions (TV: Television), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile telephones, mobile gaming devices, and other small terminal devices. These liquid crystal display devices can be generally classified as transmissive, reflective, or transflective (using transmitted light and reflected light jointly) according to the type of light source used. Energy consumption can be reduced in the reflective type, since it can utilize external light in the display, but contrast and other aspects of display performance are inferior compared to the transmissive type. Therefore, transmissive and transflective liquid crystal display devices are currently in the mainstream. In transmissive and transflective liquid crystal display devices, a light source device is installed on the back surface of a liquid crystal panel, and a display is created using the light emitted by the light source device.
Since a display device mounted in a large terminal device is most often viewed by numerous users at once, the light source device used in the display device is preferably capable of producing a uniform luminance distribution in a wide range. In contrast, a display device mounted in a medium- or small-sized terminal device is often viewed by a single user, or a small number of users at once, so the light source device used therein may radiate light in the frontal direction only. Attempts have therefore been made to increase directivity in the frontal direction and reduce power consumption in a light source device that is mounted in a medium- or small-sized terminal device, and some of these light source devices are in practical use.
FIG. 28 is a schematic perspective view showing the first conventional light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIG. 28, the first conventional light source device is composed of a light source 1101, an optical waveguide 1102 for propagating and emitting in planar fashion the light emitted by the light source 1101, a diffusing sheet 1103 disposed on the side of the light-exiting surface of the optical waveguide 1102, two prism sheets 1104 and 1105 disposed on the diffusing sheet 1103, a diffusing sheet 1106 disposed on the prism sheets, and a reflecting sheet 1107 disposed on the opposite side from the light-exiting surface of the optical waveguide 1102. A dot shape is printed on the surface of the optical waveguide 1102. A prism shape in a one-dimensional arrangement extending in one direction is formed in the two prism sheets 1104 and 1105. The apex angle of this prism shape is 90 degrees. The prism sheets 1104 and 1105 are also arranged so that the extension direction of the prism shape formed in the prism sheet 1104 and the extension direction of the prism shape formed in the prism sheet 1105 are orthogonal to each other. Furthermore, the prism sheets 1104 and 1105 are arranged so that the prism surfaces face upwards (the side opposite the optical waveguide).
In the first conventional light source device that has this type of configuration, the light emitted from the light source 1101 enters the optical waveguide 1102 from the side surface thereof, and is propagated in the optical waveguide 1102. A portion of the light is then scattered by the printed dot pattern and emitted from the emitting surface of the optical waveguide 1102. The uniformity ratio of illuminance of the light emitted from the optical waveguide 1102 is enhanced by the diffusing sheet 1103 disposed between the optical waveguide 1102 and the prism sheet 1104, and the light enters the prism sheets 1104 and 1105. Since the apex angle of the prism sheets 1104 and 1105 is 90 degrees, light rays directed near a 30-degree angle from the front refract and proceed in the front direction. As a result, the light rays are focused in the front direction, and the frontal luminance is enhanced.
FIG. 29 is a schematic perspective view showing the second conventional light source device cited on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIG. 29, the second conventional light source device is composed of a light source 2101, an optical waveguide 2102 for propagating and emitting in planar fashion the light emitted by the light source 2101, a prism sheet 2103 disposed on the side of the light-exiting surface of the optical waveguide 2102, and a reflecting sheet 2104 disposed on the opposite side from the light-exiting surface of the optical waveguide 2102. The optical waveguide 2102 is a matt prism optical waveguide in which a matte pattern (not shown in the drawing) is formed on the light-exiting surface thereof, and a row of prisms extending in the direction orthogonal to the extension direction of the light source is formed on the surface facing the reflecting sheet 2104, which is the surface on the opposite side. The prism sheet 2103 is arranged with the prism surface towards the side of the optical waveguide, the extension direction of the prism rows is the direction that is parallel to the extension direction of the linear light source, and the arrangement direction of the prism rows is the direction that is orthogonal to the extension direction of the light source.
In the second conventional light source device that has this type of configuration, the light emitted from the light source 2101 enters the optical waveguide 2102 and is propagated in the optical waveguide 2102. A portion of the light is then excluded from the condition of total reflection by the matt pattern formed in the light-exiting surface, which is the surface on the side of the prism sheet of the optical waveguide 2102, and is emitted from the optical waveguide 2102. The light emitted from the optical waveguide 2102 is in a condition slightly removed from the total reflectance condition of the optical waveguide 2102, and is therefore highly directed light having a peak near 65 degrees from the normal to the emitting surface in the direction orthogonal to the light source. This light enters the prism sheet 2103, but is totally reflected by the tilted surface of the prism on the opposite side and emitted in the frontal direction after being refracted by the tilted surface of the prism on the incident side.
As previously mentioned, since the light that is incident on the prism sheet 2103 has high directivity in the direction orthogonal to the light source, the light emitted from the prism sheet also has high directivity with respect to the direction orthogonal to the light source. On the other hand, directivity in the direction parallel to the light source is ensured by forming a row of prisms extending in the direction orthogonal to the light source in the surface on the side of the reflecting sheet 2104 of the optical waveguide 2102. FIGS. 30A and 30B are graphs showing the results of comparing the directivity characteristics of the second conventional light source device with the directivity characteristics of the first conventional light source device, wherein the horizontal axis represents the exit angle, and the vertical axis represents the light intensity. FIG. 30A shows the directivity in the vertical direction, and FIG. 30B shows the directivity in the horizontal direction. FIGS. 30A and 30B show what is described in FIG. 14 on pages 14 through 21 of the April 2004 issue of Monthly Display. As shown in FIGS. 30A and 30B, in the second conventional light source device, the directivity is increased not only in the direction orthogonal to the light source, but also in the parallel direction, and directivity is increased more than by the first conventional light source device.
FIG. 31 is a schematic perspective view showing the third conventional light source device shown in FIG. 21 of Japanese Laid-Open Patent Application 9-265092, and FIG. 32 is a partial enlarged view showing area A in FIG. 31. As shown in FIG. 31, the third conventional light source device is composed of a light source 3101; an optical waveguide 3102 for propagating and emitting in planar fashion the light emitted by the light source 3101; a propagation direction characteristic correcting element 3114 for correcting the light emission direction, disposed on the side of the light-emitting surface of the optical waveguide 3102; a reflecting pattern sheet 3116 for performing limited transmission of the light rays incident on the propagation direction characteristic correcting element 3114, disposed between the optical waveguide 3102 and the propagation direction characteristic correcting element 3114; and a reflector 3103 disposed on the opposite side from the light-emitting surface of the optical waveguide 3102. The optical waveguide 3102 is an optical waveguide having scattering properties in which a silicone-based resin material as a substance having a different refractive index is uniformly mixed and dispersed in a matrix composed of polymethyl methacrylate (PMMA), and is formed in a wedge shape whose thickness continuously decreases in the direction away from the light source. The light source 3101 is also disposed on the side where the optical waveguide 3102 is thickest.
As shown in FIG. 32, the propagation direction characteristic correcting element 3114 is provided with a plurality of conical protrusions 3114c, and the conical protrusions 3114c are convex elements that form a two-dimensional array. Flat areas 3114g are formed on the distal ends of the conical protrusions 3114c. The flat areas 3114g are parallel to the light-emitting surface of the optical waveguide 3102. The reflecting pattern sheet 3116 is formed from PMMA or another transparent resin material, for example, and a reflecting film 3116a that is composed of an Ag film or Al film and has mirror reflecting properties is provided to the surface on the side of the propagation direction characteristic correcting element 3114. Circular or elliptical openings are formed in the reflecting film 3116a, and these openings serve as windows 3116w. The portion not occupied by the windows 3116w in the reflecting film 3116a, specifically, the portion in which the Ag film or Al film is formed, is the reflecting portion 3116r. The arrangement period of the windows 3116w in the reflecting pattern sheet 3116 is the same as that of the convex elements 3114c of the propagation direction characteristic correcting element 3114. The two-dimensional positioning of the dimensions of the reflecting portion 3116r and the convex elements (conical protrusions) 3114c relative to each other is designed so as not to obstruct entrance of light into the flat areas 3114g, and so as to inhibit entrance of light into the valleys between the flat areas 3114g. 
In the third conventional light source device thus configured, the light emitted from the light source 3101 is incident on and propagated in the optical waveguide 3102, and a narrowly directed luminous flux from the light-emitting surface of the optical waveguide 3102 is emitted at an angle of about 60 to 80 degrees from the normal of the light-emitting surface. The direction in which this luminous flux is propagated has a certain spread both in the cross-section in the direction orthogonal to the lamp and in the cross-section in the direction parallel to the lamp. The luminous flux emitted from the optical waveguide 3102 enters the reflecting portion 3116r or the windows 3116w after being transmitted through the reflecting pattern sheet 3116. The luminous flux incident on the reflecting portion 3116r is reflected towards the inside of the reflecting pattern sheet 3116. Reflection by the inside surface of the reflecting pattern sheet 3116, reflection by the light-emitting surface of the optical waveguide 3102, re-entry into the optical waveguide 3102, re-exiting from the light-emitting surface, and the like provide opportunities for this reflected light and the small amount of reflected light from the windows 3116w to re-enter the windows 3116w. The reflected light is thereby recycled and reused.
Most of the light that is directly incident on the windows 3116w or that enters the windows 3116w through the recycling process described above passes through the windows 3116w and enters the flat areas 3114g of the conical protrusions 3114c at an angle. The parallelism of the propagation direction of the luminous flux is improved when the luminous flux enters the flat areas 3114g at an angle. Incidence on the valleys (null areas) between the flat areas 3114g is suppressed by the reflecting pattern sheet 3116. The luminous flux that is incident at an angle on the flat areas 3114g of the propagation direction characteristic correcting element 3114 is reflected by any portion of the peripheral surfaces 3114s of the conical protrusions 3114c, and is focused in the nearly frontal direction. This reflection causes no reduction in the parallelism of the luminous flux. The luminous flux that is focused in the nearly frontal direction is emitted in substantially orthogonal fashion from the light-emitting surface of the propagation direction characteristic correcting element 3114. It is thereby possible to obtain a light source device that is capable of endowing the emitted light with directivity in two dimensions.
However, the conventional techniques described above have such problems as the following. In the first conventional light source device, the luminous flux emitted from the optical waveguide 1102 and endowed with more uniform luminance by the diffusion sheet 1103 is refracted by two orthogonally arranged prism sheets 1104 and 1105. As a result, the directivity in the frontal direction is increased, and the two prism sheets refract light rays directed near 30 degrees from the normal to the frontal direction, but light rays at other angles are refracted or totally reflected in directions other than the frontal direction. The increase in directivity characteristics is therefore limited. A plurality of prism sheets is also needed in order to increase the directivity in two dimensions, leading to increased thickness of the light source device. This increase in thickness is a particularly significant problem when the light source device is installed in a mobile terminal device.
Unlike the first conventional light source device, the light source device can be made thinner since the second conventional light source device uses a single prism sheet. Higher directivity can be achieved than in the first conventional light source device in the direction perpendicular to the light source, since the luminous flux having high directivity emitted from the optical waveguide is emitted in the frontal direction using the total reflection of the prism sheet. Directivity is increased in the direction parallel to the light source by a row of prisms that is provided to the side of the reflecting sheet of the optical waveguide and is directed perpendicular to the light source, but directivity in the direction perpendicular to the light source is low, and the increase in the two-dimensional directivity is limited.
Furthermore, in the third conventional light source device, two-dimensional directivity is achieved by the reflecting pattern sheet 3116 that has a plurality of circular openings and by the propagation direction characteristic correcting element 3114 that has a plurality of conical protrusions. High directivity can be obtained in the aforementioned second conventional light source device because the principle of total reflectance designed to enhance directivity in the direction orthogonal to the light source is used in two dimensions. However, this method has the significant problem of increasing the thickness of the light source device, since a reflecting pattern sheet and an element for correcting the propagation direction characteristics are required.